Re-writable optical disk having reference clock information permanently formed on the disk

ABSTRACT

An optical disk structure and optical disk recorder which enables data to be rewritten onto the recording layer of the optical disk. A clock reference structure is permanently formed along servo tracks of the optical disk. An optical transducer is coupled to the clock reference structure and generates a clock reference signal simultaneously with writing new data onto the recording layer of the optical disk. The data is written as data marks along the servo tracks. Each of the data marks includes edges. The edges of the data marks are recorded in synchronization with a write clock. The write clock is phase-locked with the clock reference signal. Therefore, the edges of the data marks are aligned with the clock reference structure with sub-bit accuracy. Standard DVD-ROM disk readers are not able to detect the high spatial frequency of the clock reference structure. Therefore, the optical disk structure and optical disk recorder of this invention allow production of re-writable optical disks which can be read by standard DVD-ROM disk readers.

CROSS REFERENCE TO RELATED APPLICATIONS

Notice: More than one reissue application has been filed for the reissueof U.S. Pat. No. 6,046,968. The reissue applications are applicationSer. No. 13/622,833, which is a continuation reissue of application Ser.No. 12/725,099, now RE43,788, which is a divisional reissue ofapplication Ser. No. 09/997,036, now RE41,881, which is a reissue ofU.S. Pat. No. 6,046,968.

FIELD OF INVENTION

This invention relates generally to storing data on re-writable opticaldisks. In particular, it relates to an optical disk having servo tracksincluding a clock reference structure for generating a clock referencesignal for accurately controlling the placement of data marks along theservo tracks when writing information to a recording layer of theoptical disk, and an optical disk recorder for writing the informationto the optical disk.

BACKGROUND

Typically, data is stored on a recording layer of an optical disk byforming either data pits or data marks on the recording layer of thedisk. The data pits or marks are formed along servo tracks on therecording layer of the optical disk. A servo track is a permanentphysical feature on the recording layer of the optical disk whichprovides a track-following reference and defines the path along whichstat is written. Servo tracks may be spiral or concentric. A groove isan example of a servo track. In some types of prerecorded optical disks,such as read only memory (ROM) disks, the data pits formed on therecording layer also function as a servo track.

Typically, an optical transducer which includes a focused laser beam iscoupled to a servo track on the recording layer of the optical disk.When reading the optical disk, the data pits or marks formed along theservo track pass by the optical transducer as the optical disk rotates,causing the optical transducer to generate data signal representing thedata stored on the recording layer of the disk. The optical transducerincludes a focus positioner and a tracking positioner for maintainingalignment of the focused laser beam with respect to the servo track inthe focus direction and the cross-track direction as the optical diskrotates. The focus and tracking positioners include servo-controlsystems which respond to focus and tracking error signals produced bythe optical transducer.

FIGS. 1a, 1b, 1c show a typical mass-produced optical ROM disk in whichprerecorded data is stored on an optical disk 10 by forming apredetermined series of data pits 12 along a track 14 of the opticaldisk 10. FIG. 1a shows a top-view of the optical disk 10. FIG. 1b showsan expanded view of the track 14 shown in FIG. 1a. FIG. 1c shows across-sectional view of the track 14 shown in FIG. 1b. The recordinglayer of the optical disk 10 is permanently formed during manufacturingto create the data pits 12. Therefore, data on an optical disk 10 whichis stored by forming data pits 12 on the recording layer of the opticaldisk 10 can not be erased or re-written.

In a re-writable optical disk, such as a phase change optical disk, datais stored on the recording layer of the optical disk in the form of datamarks by controlling the optical characteristics of the recording layerof the disk. Data marks are formed on the recording layer by heating therecording layer of the disk with a focused laser beam at the locationswhere the data marks are to be written. In phase change recording, theoptical reflectivity of the data mark is determined by the crystallinecondition of the recording layer. The crystalline condition of therecording layer is determined by controlling the optical power in thefocused laser beam. The optical power of the laser beam used to heat therecording layer determines the rate at which the temperature of therecording layer of the optical disk cools where the data mark islocated. The rate at which the data mark location of the recording layercools determines whether the location cools to an amorphous or acrystalline condition. Typically, the recorded data mark is amorphousand the surrounding area is crystalline.

FIGS. 2a, 2b, 2c show a re-writable optical disk 20 in which data isstored on the optical disk 20 by forming a series of data marks 22 alonga track 24 of the optical disk 20. FIG. 20a shows a plan-view of theoptical disk 20. FIG. 2b shows an expanded view of the track 24 shown inFIG. 2a. FIG. 2c shows a cross-sectional view of the track 24 shown inFIG. 2b.

In the prior art, placement of data to be written on a recording layerof a re-writable optical disk is typically determined by includingsynchronization information between fixed-length data fields. A sectoris a repeating unit of pre-determined length. FIG. 3a shows a plan-viewof a prior art optical disk 30 in which data stored along a servo track32 is divided into sectors 34. FIG. 3b shows an expanded view of asector 34 of the optical disk shown in FIG. 3a. The sector 34 includes aheader 36, a data field 38 having a predetermined length, and an editgap 40. FIG. 3c shows an expanded view of the header 36 shown in FIG.3b. The header 36 includes synchronization information 42 and trackaddress information 44. The synchronization information 42 is alsoreferred to as the sync field. The synchronization information 42 ispermanently encoded on the recording layer of the optical disk 30 withinthe sectors 34. Data written onto the recording layer of the opticaldisk 30 is synchronized to a write clock. The write clock issynchronized to a clock reference signal which is generated periodicallyas the synchronization information 42 passes by the optical transduceras the optical disk 30 rotates. The clock reference signal providesposition information of the optical transducer with respect tosynchronization information 42 on the recording layer of the opticaldisk 30 when the synchronization information 42 passes by the opticaltransducer. However, while data within data fields 38 is being writtenby the optical transducer, the clock reference signal drifts infrequency and phase. That is, when the optical transducer is betweenpoints where synchronization information 42 exists, the frequency andphase of the write clock can drift with respect to the synchronizationinformation 42 located within sectors 34. Drift of the write clock withrespect to the synchronization information 42 can be caused by diskrotation speed variations, servo track eccentricity and the cumulativeeffect of other variations in an optical disk recorder such as clockfrequency drift. In general, the greater the distance between syncfields, the greater the drift of the write clock.

The edit gap 40 shown in FIG. 3b is included within the sector 34. Adata field which includes a fixed number of data bits is typicallywritten to the sector 34 of the recording layer of the optical disk 30.The edit gap 40 accommodates variations in the placement of the lastdata bit of a data field which is written to the sector 34. That is,although all data fields normally contain the same number of data bits,the edit gap 40 allows the placement of the last data bit of a datafield to be different each time the data field is re-written. Therefore,placement of bits written to the recording layer is not required to beas precise as the placement would be required to be if the edit gap 40did not exist. Edit gap are needed to accommodate drift of the writeclock in prior art re-writable optical disks.

Presently existing DVD read only memory (ROM) formats do not includephysical sectoring of data stored on the recording layer of an opticaldisk. Therefore, synchronization fields and edit gaps are not provided.When reading a ROM optical disk, a read clock is produced from the datastored on the optical disk. Therefore, no synchronization information isrequired.

The DVD read only memory (ROM) format specification organizes data intofixed-length data field for error correction code (ECC) purposes. Eachdata field has an associated header containing synchronization andaddress information to facilitate data readout. This synchronization andaddress information is stored on the disk in the form of data pits whichare indistinguishable from the data pits used to encode data. Although aDVD-ROM data field, together with its associated header informationmakes up a “physical sector” for the purposes of a read-only memory, itdoes not satisfy the requirements of a physical sector for the purposesof a re-writable optical disk memory. For this reason, all sectoring ofthe DVD format is treated as “logical sectoring.” A logical sector iscontained within the data, whereas a physical sector contains the data.Therefore, all synchronization information, addressing and other DVDformatting are treated as if they were data, and are written on the diskin the form of data marks at the same time data is written.

Writing data to the recording layer of a re-writable optical disk whichis compatible with DVD-ROM formats therefore requires the data to bewritten to a disk having no physical sectors on the unwritten disk, andtherefore, no address or synchronization information in dedicated areswithin the physical sectors. Furthermore, edit gaps can not be included.Without edit gaps, the data marks must be written with sub-bit accuracyduring overwriting of pre-existing data.

U.S. Pat. Nos. 4,238,843, 4,363,116, 4,366,564, 4,375,088, 4,972,401teach methods of permanently providing additional synchronizationinformation along the tracks of an optical disk within data fields. Theteachings of these patents also include synchronization informationwithin sync fields between the data fields. Further, the spatialfrequency of the synchronization information which is within the datafields must correspond with nulls in the spatial frequency of the data.This requires the data to be encoded using special codes so that nullsin the spatial frequency of the data correspond with the spatialfrequency of the synchronization information.

It is desirable to have a re-writable optical disk and an optical diskrecorder capable of recording data on the optical disk wherein therecorded disk is compatible with DVD-ROM standard formats, and isreadable by a standard DVD reader, and wherein pre-existing data on theoptical disk can be re-written (sometimes called over-written) with newdata with sub-bit accuracy. The optical disk and optical disk recordermust be capable of generating a write clock which is synchronized withsub-bit accuracy to absolute position along the servo tracks of theoptical disk. Further, it is necessary to be able to write standard DVDdata formats.

The present invention provides a re-writable optical disk having arecording layer which includes a permanent clock reference structureprovides a clock reference signal generated by an optical transducer asthe clock reference structure passes by the optical transducer as theoptical disk rotates. A write clock is phase-locked to the clockreference signal. The write clock enables new data to be written to therecording layer of the optical disk with sub-bit accuracy. Further, thewrite clock eliminates the need for sync fields and edit gaps andprovides a means for writing and re-writing data in data fields ofindeterminate length.

A first embodiment of the invention includes an optical disk. Theoptical disk includes a recording layer having servo tracks. A clockreference structure is formed along the servo tracks. The clockreference structure permits data to be written to the recording layer indata fields of indeterminate length. The clock reference structurecomprises a reference spatial frequency which is greater than apredetermined spatial frequency. An extension of this embodimentincludes the predetermined spatial frequency being greater than themaximum spatial frequency detectable by a standard DVD-ROM reader.

Another embodiment of the invention includes an optical disk recorder inwhich an optical disk is rotatably mounted on the recorder. The opticaldisk includes a recording layer containing servo tracks. An opticaltransducer radially follows a servo track as the optical disk rotates. Aclock reference structure pre-exists along the servo tracks and providesdata fields of indeterminate length. The clock reference structurecauses the optical transducer to produce a clock reference signal as theoptical disk rotates. The optical disk recorder further includes a meansfor recording data marks on the recording. layer of the optical disk.The data marks are recorded so that a standard DVD-ROM reader can readthe data marks but the optical disk is constructed so that the readercannot detect the clock reference structure. A write clock determinesthe physical placement of data marks written on the recording layer ofthe optical disk. The write clock is phase locked to the clock referencesignal.

Another embodiment of the invention includes an optical disk recorderfor receiving an optical disk. The optical disk is rotatably mountableon the recorder. The optical disk includes a recording layer havingservo tracks and a clock reference structure having a spatial frequencywhich is too high to be detected by a standard DVD-ROM reader. The clockreference structure is formed along the servo tracks and provides datafields of indeterminate length. The optical disk recorder includes anoptical transducer which is optically coupled to the recording layer ofthe optical disk. The optical transducer follows the servo tracks of theoptical disk as the optical disk rotates. The clock reference structureformed along the servo tracks of the optical disk causes the opticaltransducer further includes a means for writing data marks on therecording layer of the optical disk. A write clock determines thephysical placement of data marks written on the recording layer of theoptical disk. The write clock is phase locked to the clock referencesignal.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a plan-view of a prior art ROM optical disk.

FIG. 1b shows a expanded view of a track shown in FIG. 1a.

FIG. 1c shows a cross-sectional view of the track shown in FIG. 1b.

FIG. 2a shows a plan-view of a prior art re-writable optical disk.

FIG. 2b shows an expanded view of a track shown in FIG. 2a.

FIG. 2c shows a cross-sectional view of the track shown in FIG. 2b.

FIG. 3a shows a plan-view of a prior art optical disk 30 in which datastored along a servo track 32 is divided into sectors 34.

FIG. 3b shows an expanded view of a sector 34 of the optical disk shownin FIG. 3a.

FIG. 3c shows expanded view of the header 36 shown in FIG. 3c.

FIG. 4a, 4b, 4c shows the components of various types of sectors on anoptical disk.

FIG. 5a shows a prior art embodiment of a low frequency referencestructure for encoding address information.

FIG. 5b shows a high frequency clock reference structure.

FIG. 5c shows a structure obtained by combining the structures of FIG.5a and FIG. 5b.

FIG. 6a shows a plan-view of an embodiment of the invention.

FIG. 6b shows an expanded view of the servo tracks of the optical diskshown in FIG. 6a including data marks.

FIG. 7 shows a cross-sectional view of the servo tracks of the opticaldisk shown in FIG. 6a.

FIG. 8 shows a relationship between the edges of data pits and a channelbit length.

FIG. 9a shows a clock reference structure formed along a servo track.

FIG. 9b shows a clock reference signal produced by the clock referencestructure of FIG. 9a.

FIG. 9c shows a square wave clock reference signal formed byelectronically processing the clock reference signal of FIG. 9b.

FIG. 9d shows a write clock formed by electronically processing thesignal of FIG. 9c.

FIG. 10 shows an embodiment of an optical disk and an optical diskrecorder in accordance with the invention.

FIG. 11 shows the modulation transfer function (MTF) of an opticaltransducer of an optical disk recorder constructed in accordance withthe invention, and the MTF of a standard DVD optical disk reader.

FIG. 12 shows an embodiment of the electronic control circuitry used togenerate the write clock from the clock reference signal.

FIG. 13 shows the frequency spectrum of a prior art optical diskstructure in which the clock reference structure has a spatial frequencywhere the spatial frequency of the data has been specifically nulled.

FIG. 14 shows the frequency spectrum an embodiment of the invention inwhich the clock reference structure has a spatial frequency which isgreater than the spatial frequency spectrum of the data.

FIG. 15 show the frequency spectrum of another embodiment of theinvention in which the clock reference structure has a spatial frequencywhich overlaps the spatial frequency spectrum of the data.

FIG. 16 shows a clock reference structure consisting of pits formedalong a servo track.

FIG. 17 shows a quadrant detector used to produce clock referencesignals.

FIG. 18 shows MTF curves for split detection and push pull detection.

FIG. 19 shows servo tracks having a clock reference structure consistingof groove edges that oscillate substantially 180 degrees out of phase.

FIG. 20 shows servo tracks having a clock reference structure consistingof groove edges that oscillate in phase.

FIG. 21 shows an embodiment of the invention having a second opticaltransducer for reading data.

FIG. 22 shows an embodiment of the invention having anotherconfiguration of a second optical transducer which shares a combinationobjective lens with a first optical transducer.

DETAILED DESCRIPTION

As shown in the drawings for purposes of illustration, the invention isembodied in an optical disk structure and optical disk recorder whichenables data to be written or re-written onto the recording layer of theoptical disk. The data can be written or re-written with sub-bitaccuracy without requiring the unwritten optical disk to be divided intophysical sectors. Furthermore, synchronization fields and edit gaps arenot required on an optical disk on which data is to be written. Theoptical disk recorder includes an optical transducer which can resolveand detect a high spatial frequency clock reference structure located onthe optical disk. Standard DVD-ROM disk readers are not able to resolveand detect the clock reference structure. Therefore, the optical diskstructure and optical disk recorder of this invention allow productionof re-writable optical disks which can be read by standard DVD-ROM diskreaders. Additionally, the optical disk recorder can read optical disks.

A clock reference structure is permanently formed along servo tracks ofthe optical disk. An optical transducer is coupled to the clockreference structure and generates a clock reference signalsimultaneously with writing new data onto the recording layer of theoptical disk. The data is written as data marks along the servo tracks.Each of the data marks includes a first and a second edge. Duringrecording, the edges of the data marks are formed in synchronizationwith a write clock. Therefore, each time the edge of a data mark isformed, the write clock had completed the dame fraction of a cycle. Thewrite clock is phase-locked to the clock reference signal. Therefore,the edges of the data marks are formed in synchronization with the clockreference signal with sub-bit accuracy. Therefore, the edges of the datamarks are accurately aligned with the clock reference structure. Theedge of the data mark is only recorded when required by the data writtenand the data encoding scheme. Many cycles of the clock referencestructure will not have a corresponding data mark edge.

FIGS. 4a, 4b, 4c illustrate a comparison of the fields of informationalong the tracks of two prior art optical disks, and the data fields ofthe present invention. FIG. 4a shows a prior art sector which includes async field 42, a servo field 46, an address field 44, a data field 38and an edit gap 40. FIG. 4b shows a prior art sector which includes async field 42, an address field 44, a data field 38 and an edit gap 40.FIG. 4c shows a data field 50 of the invention. The present inventiondoes not require a sync field, an address field, a servo field or anedit gap. Furthermore, the data field 50 of the invention is arbitraryin length. The clock reference structure provides synchronizationinformation which is precise enough to allow elimination of edit gaps.The track address information is included within the clock referencestructure.

As shown is FIG. 4c, the data fields of the invention are indeterminatein length. A data field of indeterminate length is a data field in whichthe data field length and corresponding data capacity are not determinedby any permanent structure formed on the optical disk. Therefore, thelengths of data blocks can be determined solely by the requirements ofthe format and the code used to record the data. Many data formats,including the DVD format, establish uniform data field lengths andinclude addresses and synchronization information for use during dataread out. This information is recorded as data marks. A data field ofindeterminate length can accommodate any pattern of data marks in anycode or format, whether it represents synchronization information,address information, data or other information. Any information whichcan be written as data marks can be written anywhere on the disk. In thecase of spiral servo tracks, one data field of indeterminate length canbe as large as the entire disk. In the case of circular servo tracks,one data field of indeterminate length can be as large as one entiretrack.

The track address information can be included as a lower spatialfrequency modulation superimposed on the higher clock referencestructure spatial frequency. FIG. 5a shows a low spatial frequency trackaddress structure 52 which includes track address information. FIG. 5bshows a high spatial frequency clock reference structure 54. FIG. 5cshows the structures of FIG. 5a and FIG. 5b combined. The structure ofFIG. 5c provides both a clock reference structure for generating a clockreference signal and a track address structure for generating a trackaddress signal. Alternatively, the track address information canmodulate the spatial frequency of the clock reference structure.

A purpose of the present invention includes the elimination of physicalsector information from unrecorded optical disks. The term “physicalsectors” for the purposes of the description of the invention refers topermanently embossed structures between the data fields shown in FIG. 4aand FIG. 4b. For the invention, synchronization information can be usedto synchronize a clock within an optical disk reader to read data withina data field. Such synchronization information is not present beforedata has been written to the optical disk. Further, such synchronizationinformation is not required by an optical disk recorder of the inventionto generate a reference clock signal. The DVD-ROM specification refersto the segmenting of data and the inclusion of synchronizationinformation within the data as “physical sectoring. ” For thedescription of the invention, this is referred to as “logical sectoring”to distinguish it from permanently embossed synchronization informationlocated between data fields which is formed during manufacturing of are-writable optical disk.

FIGS. 6a, 6b show an embodiment of the invention. FIG. 6a shows aplan-view of an optical disk 50 of this embodiment. This embodimentincludes the optical disk 50 having servo tracks 52, 54, 56. FIG. 6bshows an expanded view of the servo tracks 52, 54, 56 shown in FIG. 6a.Each of the servo tracks 52, 54, 56 includes data marks 58 which arewritten to a recording layer of the optical disk 50. Each of the servotracks 52, 54, 56 are shaped as grooves which include a first edge 60and a second edge 62. FIG. 7 shows a cross-sectional view of the servotracks 52, 54, 56. The first edge 60 and the second edge 62 are formedto oscillate at a predetermined spatial frequently and phase. For thisembodiment, the first edge 60 and the second edge 62 oscillate in-phase.The data marks 58 are formed in the grooves of the servo tracks 52, 54,56.

Alternate configurations of the embodiment shown in FIGS. 6a, 6b exist.For example, the data marks 58 may be written to the recording layer inthe grooves or between the grooves. The data marks 58 may be permanent(write once) or re-writable. The data marks 58 may affect the amplitude,phase or polarization of light emitted from an optical transducer. Theservo tracks 52, 54, 56 may be concentric or spiral.

Another embodiment of the invention includes the data marks 58 beingformed on the recording layer by heating the recording layer of theoptical disk 50 with a focused laser beam at the locations where thedata marks 58 are written. In phase-change recording, the opticalreflectivity of the data marks 58 is determined by controlling the rateat which the temperature of the recording layer of the optical disk 50cools where the data marks 58 are located.

The recording layer of an optical disk is characterized by a recordingthreshold. The recording threshold being the minimum irradiance (opticalpower per unit area) at the recording layer necessary to alter therecording layer in an optically detectable way; for example by writingdata marks. Irradiance levels below the recording threshold do not alterthe recording layer and are used for producing the focus and trackingerror signals used for maintaining the alignment of the opticaltransducer with the servo track. Irradiance levels below the recordingthreshold are also used in an optical disk reader to read recorded data.

As is well known in the art, the optical power emitted by a laser diodecan be modulated at very high frequencies by modulating the electricalcurrent used to drive the laser diode. Data recording is accomplished bymodulating the laser diode drive current, thereby modulating the opticalpower emitted by the laser and, consequently, the irradiance at therecording layer. Whenever the irradiance at the recording layer ismodulated above the recording threshold the recording layer is alteredand a data mark is written. The positions of edges of data marks alongthe servo track correspond with the times of read signal transitionswhen the data marks are read by an optical disk reader.

Methods for manufacturing grooved optical disks are well known in theart and are currently employed in the production of most re-writableoptical disks. Typically, a smooth glass disk is coated with photoresistand exposed with a focused laser beam as the disk is rotated on aprecision spindle under servo control. For a spiral groove, the focusedlaser beam is continuously translated in the radial direction as thedisk rotates. The exposed disk is developed to remove exposedphotoresist and to harden unexposed photoresist; the exposed glass diskis then called the master. The master is then heavily plated with ametal (usually nickel), filling the grooves where the photoresist wasexposed by the laser. The metal plating is separated from the master andmounted to a metal backing plate to form a sub-master or stamper. Thesub-master is used as one surface of a mold used to manufacture grooveddisk substrates. Substrates are normally injection molded formtransparent polycarbonate plastic and then coated with the recordinglayer to form re-writable optical disks. The recording layer is thencoated with a protective lacquer film. Laser light used for reading andwriting data is focused through the substrate. This arrangement protectsthe recording layer from damage and contamination.

The edges of the grooves can be formed to oscillate in-phase radiallydeflecting the laser beam while exposing the photoresist in themastering process. As is well known in the field of optics, highfrequency deflections are practical to implement using a galvanometermirror, an electro-optic deflector or an acousto-optic deflector in thepath of the laser beam between the laser and the objective lens.

The edges of the grooves can be formed to oscillate substantially 180degrees out-of-phase by modulating the power of the laser beam whileexposing the photoresist in the mastering process. Numerous practicalmethods exist for modulating the power of a laser beam at a highfrequency. Some lasers can be modulated directly by controlling acurrent or voltage source connected to the laser. Otherwise, anelectro-optic or acousto-optic modulator, can be used in the laser beampath. A variety of modulation methods are also available that operatewithin the cavity of a gas laser, as is well known in the field ofoptics. Other methods of forming grooves can be used including a methodwhich forms a clock reference structure which consists of a groovehaving a single edge which oscillates.

Prior to writing data to an optical disk, the data is encoded. A primarypurpose of encoding data is to maximize the data storage capacity of thedisk. Using the DVD format as an example, FIG. 8 illustrates some of theelements of a data encoding scheme. Data pits 31 are formed along servotrack center line 35. The shortest pit 37 that can be reliably read by aDVD reader has a length 39 equal to 0.40 um. This is dimension isapproximately equal to the width of all pits, and the shortest pit 37 istherefore nearly circular. The shortest readable distance 41 betweenadjacent pits is also 0.40 um. The lengths of pits and the lengths ofspaces between pits are required by the code to be integer multiples ofa channel bit length 43. The distance between any two pit edges istherefore an integral multiple of the channel bit length. The channelbit length for the DVD code is 0.133 um. The pit lengths and spacelengths allowed by the DVD code ar 0.400, 0.553, 0.666 . . . 1.866 um.The shortest pit is 3 channel bits long (0.400 um) and the longest pitis 14 channel bits long (1.866 um).

FIGS. 9a, 9b, 9c, 9d show how a write clock of the invention isproduced. A servo track comprises a groove 3 on the recording layer ofan optical disk. A clock reference structure comprises groove edges 5and 7 which oscillate substantially 180 degrees out of phase. As theclock reference structure passes the optical transducer (not shown) ofan optical disk recorder (not shown) the optical transducer producesclock reference signal 9 shown in FIG. 9b. As the optical disk rotates,the clock reference structure passes by the optical transducer. In FIG.9a, an increment of disk motion equal to one spatial period 11 of theclock reference structure causes the clock reference signal 9 of FIG. 9bto pass through one temporal period 13 of modulation. As shown in FIGS.9a, 9b, 9c, 9d, one spatial period 11 of the clock reference structurehas a length equal to four channel bits. One temporal period 13 ofmodulation of the clock reference signal 9 has time duration equal offour cycles of the write clock. Thus, one cycle of the write clockcorresponds to one channel bit length on the disk For purposes ofillustration, the spatial period 11 of the clock reference structure ofFIG. 9a is shown at the same drawing scale as a corresponding incrementof time. Namely, the temporal period 13 of the clock reference signalshown in FIG. 9b. The ratio of an increment of disk motion to thecorresponding increment of time is the velocity of the disk.

Clock reference signal shown in FIG. 9b is electronically processed toproduce a square wave clock reference signal 15 shown in FIG. 9c. Eachcycle of the square wave clock reference signal 15 is electronicallydivided by four to produce a write clock 17 as shown in FIG. 9d. Writeclock 17 is a temporal signal produced at four times the frequency ofthe clock reference signal and is phase locked to the clock referencesignal. Write clock 17 remains phase synchronized with the clockreference signal regardless of the rotational speed of the disk.

In FIG. 9a, edges of data marks 19 are spatially aligned with the clockreference structure and are therefore temporally aligned with the clockreference signal. Data marks 19 which pre-exist on the disk do notaffect the process of generating the write clock and will beover-written with new data. When recording re-writable data on a DVDformat disk, a write clock is needed which has a temporal period whichcorresponds to a channel bit length of 0.133 um. Thus, one channel bitlength passes the optical transducer during each period of the writeclock. Since a clock reference structure with a spatial period of 0.133um can not be resolved by currently available optical transducers, thespatial period of the clock reference structure is selected to be amultiple of the channel bit length. In this example, one period of theclock reference structure has a length of four channel bits, equal to0.533 um. The frequency of the clock reference signal is then ¼ of thechannel bit frequency and the clock reference signal is increased infrequency by a factor of four to produce the write clock.

FIG. 10 shows an optical disk 80 and optical disk recorder 81 of thisinvention. The optical disk recorder 81 receives the optical disk 80.The optical disk recorder 81 includes a rotational motor 84 for rotatingthe optical disk 80. The optical disk recorder 81 also includes anoptical transducer 82 which generates a clock reference signal as aclock reference structure of the optical disk 80 passes by the opticaltransducer 82 as the optical disk 80 rotates. In one embodiment, thespatial frequency of the clock reference structure is greater than thespatial frequencies. detectable by standard DVD-ROM readers. Electroniccontrol circuitry 83 within the optical disk recorder 81 synchronizes awrite clock with the clock reference signal generated by the opticaltransducer 82. A write signal synchronized with the write clock controlswhen the optical transducer 82 writes edges of data marks to a recordinglayer of the optical disk 80.

The rotational motor 84 is generally the same as the rotational motorsused in prior art optical disk drives.

FIG. 10 shows that the optical transducer 82 of the optical diskrecorder 81 includes several optical devices. A laser diode 90 emits alinearly polarized beam of light 92 which is collimated by a collimatorlens 94. The light beam 92 is passed through a polarization beamsplitter 96. The light beam 92 is converted from linear polarization tocircular polarization by a quarter wave retardation plate 98. The lightbeam 92 then passes through an aperture stop 99 and is focused by anobjective lens 100 onto the recording layer of the optical disk 80whereupon data is recorded. A portion of the light beam 92 is reflectedby the optical disk 80 and returns through the objective lens 100 andthe quarter wave retardation plate 98. Upon passing back through thequarter wave retardation plate 98, the light beam 92 is again linearlypolarized. However, the polarization direction of the light beam 92 isrotated 90 degrees relative to its initial orientation. Therefore, thepolarization beam splitter 96 reflects substantially all of light beam92 towards a beam splitter 102. The beam splitter 102 splits the beam 92into a first light beam 104 and a second light beam 106. The first lightbeam 104 is collected by a first lens 108 onto a first detector 110which is arranged to produce a focus-error signal. The second light beam106 is collected by a second lens 112 onto a second detector 114 whichis arranged to produce a tracking-error signal used by the trackingpositioner, and a clock reference signal. Detectors 110 and 114generally include multiple detection areas and produce multiple signalsas is well known in the art. Many alternative arrangements of theoptical components and detectors are possible, including arrangementswhich combine or eliminate optical components shown in FIG. 10.

The optical power emitted by the laser diode 90 can be modulated at veryhigh frequencies by modulating the electrical current used to drive thelaser diode 90. Data recording is accomplished by modulating the laserdiode 90 drive current, thereby modulating the optical power emitted bythe laser diode 90, and consequently, the irradiance at the recordinglayer of the optical disk 80. The electrical current used to drive thelaser diode 90 is controlled by the write signal.

The ability of an optical system to resolve fine structures like theclock reference structure of the invention is described by themodulation transfer function (MTF) of the optical system. Using methodssuch as Fourier transforms, the spatial distribution of light leaving anobject can be represented as a spatial frequency distribution, in whicheach spatial frequency component has a particular amplitude and phase. Asimilar approach is commonly used to represent an electrical signal interms of the component temporal frequencies of the electrical signal. Anoptical system such a lens, acts as an optical filter which selectivelyattenuates each spatial frequency component in an image formed by thelens. For each spatial frequency component, the lens has a transferfactor which determines the ratio of image modulation (lens output) toobject modulation (lens input). The MTF of the lens specifies thetransfer factor as a function of spatial frequency.

An aberration-free lens such as the objective lens in the opticaltransducer of an optical disk recorder or reader, has an MTF which iswell known in the art of optics as being the appropriately scaledautocorrelation of the pupil function. FIG. 11 is a graph of the MTF fortwo aberration-free optical transducers having uniformly filled circularpupils. The functional form of the MFT is similar for allaberration-free optical transducers. However, the spatial frequency atwhich the MTF goes to zero, known as the cutoff frequency, depends onthe numerical aperture (NA) of the objective lens and the wavelength (λ)of the light forming the image. The cutoff frequency is determined by2*NA/λ, and is the highest spatial frequency that can be detected by thetransducer. Any spatial frequency component higher than the cutofffrequency will not exist in the output of the optical transducer. In anoptical disk reader or recorder, the cutoff frequency is determined bythe wavelength of the light source and the numerical aperture of theobjective lens used to focus a light beam onto the recording layer of anoptical disk. Curve 116 of FIG. 11 illustrates the MTF of an opticaltransducer having a numerical aperture of 0.60 and an operatingwavelength of 650 nm. The cutoff frequency 117 for this opticaltransducer is 1.85 cycles/um.

Manufacturers of optical disks and optical readers develop and agree tooptical data storage industry standards. These standards ensure that anyoptical disk can be read by any optical disk reader if the optical diskand the reader conform to the same industry standards. DVD is an exampleof an industry standard. The specifications for DVD define numerousparameters of both DVD disks and DVD readers. The specifications includecertain parameters of the optical transducer in the optical disk reader.These parameters include the wavelength (650 nm) and the numericalaperture (0.60) of the light beam focused on the optical disk. Curve 116represents the MTF for the optical transducer of an industry standardDVD optical disk reader. As illustrated by curve 116, and as calculatedabove, the cutoff frequency for an industry standard DVD reader is 1.85cycles/um.

The DVD standard only applies to read-only-memory (ROM). The DVDstandard does not specify the design and manufacture of re-writableoptical disks that can be read by industry standard DVD optical diskreaders. There is a market demand for re-writable DVD disks and foroptical disk recorders for writing data to the re-writable DVD disks. Anobject of this invention is to provide a continuous and permanent clockreference structure for use in recording re-writable DVD disks, whereinthe clock reference structure can not be detected by industry standardDVD readers.

Curve 118 of FIG. 11 represents the MTF of an optical transducer of anoptical disk recorder constructed according to the principles of thepresent invention. As illustrated by curve 118, the MTF of this opticaltransducer is greater at all spatial frequencies than the MTF of theoptical transducer of an industry DVD optical disk reader as illustratedby curve 116. In addition, cutoff frequency 119 for the optical diskrecorder (2.46 cycles/um) is greater than the cutoff frequency 117 of anindustry standard DVD reader (1.85 cycles/um).

Curve 118 represents the MTF of an optical transducer having a numericalaperture of 0.8 and a light beam wavelength of 650 nm. However, curve118 can alternatively represent the MTF of an optical transducer inwhich the numerical aperture is 0.6 and the light beam wavelength is 488nm. In either of these example cases, the cutoff frequency 119 is 2.46cycles/um and the shape of the MTF curve is as represented by the curve118.

FIG. 11 also shows the clock reference structure spatial frequency 121of 1.875 cycles/um. The spatial frequency of the clock referencestructure is too high for a DVD reader to detect. That is, the spatialfrequency of the clock reference structure is higher than the 1.85cycles/um cutoff frequency of an industry standard DVD reader. However,the MTF of the optical transducer of the disk recorder constructedaccording to the principles of the present invention has a cutofffrequency 119 which is greater than the spatial frequency of the clockreference structure. Therefore, the disk recorder can detect the clockreference structure and the DVD reader cannot. The numerical valuespresented here are by way of example. The principles are the same foroptical disk readers which have a higher or a lower cutoff frequency.FIG. 12 shows an embodiment of the electronic control circuitry 83 whichsynchronizes the clock reference signal with the write clock. The writesignal which is synchronized with the write clock controls when theoptical transducer 82 writes first and second transition edges of thedata marks to the surface of the optical disk 80. Generally, the writeclock functions at a frequency which is greater than the frequency ofthe clock reference signal recovered from the optical disk 80 by theoptical transducer 82.

The write clock is synchronized to the recovered clock reference signalusing a harmonic locking phase-locked loop shown in FIG. 12, anddescribed in detail in F. M. Gardner (pp 201-204, Phaselock Techniques,John Wiley & Sons, second edition, New York, N.Y., 1979).

The clock reference signal having a clock reference frequency (fr) iscoupled to the phase-locked loop through a zero crossing detector 1012.The zero crossing detector 1012 converts the clock reference signal intoa square wave. The square wave is coupled to a phase detector 1014. Thewrite clock is generated at a frequency (N*fr) by a voltage controlledoscillator (VCO) 1016. An output signal (write clock) of the VCO 1016 isfrequency divided by a frequency divider 1018. The output of thefrequency divider 1018 is coupled to the phase detector 1014. The phasedetector 1014 generates a phase detect signal in which the amplitude ofthe detect signal is proportional to the phase difference between thefrequency-divided VCO signal and the square clock reference signal.Various embodiments of the phase detector exist, some of which includecharge pump circuitry. The phase detect signal is amplified and filteredby a loop amplifier/filter 1020. The output of the amplifier/filter 1020is coupled to the VCO and will advance or retard the phase of the VCOoutput signal.

The harmonic locking phase-locked loop accomplishes two functions. Thefirst function is to generate a write clock which is at N times thefrequency (fr) of the detected clock reference signal. The secondfunction is to minimize the phase difference between the clock referencesignal and the divided VCO signal.

While there is design flexibility in the choice of the referencemultiplier N, it is not arbitrary. As noted on page 202 of Gardner; “thephase jitter at the output includes a component equal to N times thatportion of the reference jitter that passes through the loop transferfunction. Also, if there is closed-loop baseband noise vn at the phasedetector output, then the corresponding VCO jitter is N(vn/Kd) (where Kdis the gain of the VCO), assuming that the spectrum of vn lies in theloop bandwidth. If N is large, the output jitter can be unacceptable,even for respectfully small values of reference jitter on vn. Extrememeasures are sometimes needed to suppress stray circuit noises that areordinarily negligible.”

Essentially, there is a practical limitation to the size of N due to theamplification of jitter and noise in the loop. For this reason, whenwrite clock frequency greater than the clock reference frequency isrequired, it is most advantageous to maximize the clock referencefrequency. Therefore, N is minimized, which minimizes the jitter.

This creates a distinction between clock reference structures havingfundamental frequencies significantly lower than the maximum dataspatial frequency and the clock reference structure of this invention.That is, the jitter produced by clock reference structures withfundamental spatial frequencies which are significantly lower than themaximum data frequency is likely to be too great to be of practical usein writing to an optical disk unless the data is divided into sectorswhich include edit gaps within them. However, the clock referencestructure of this invention having spatial frequencies comparable to orgreater than the maximum fundamental data spatial frequency will haveless jitter and noise amplification in the harmonic lockingphase-locking loop than a clock reference structure having a spatialfrequency less than the maximum fundamental data spatial frequency.Therefore, the clock reference structure of the invention enables theproduction of a superior write clock.

The data signal and the clock reference signal are both coupled to theoptical transducer in both optical disk readers and optical diskwriters. Therefore, the data signal and the clock reference signal mustbe separated. To understand the process of separating the data signalfrom the clock reference signal, it is important to realize that as theoptical disk rotationally passes under the optical transducer at aparticular velocity, spatial frequencies of structures on the recordinglayer of the optical disk are translated into temporal frequencies. Fora given spatial frequency (υ) on the optical disk and a given linearvelocity (v) of the disk passing under the transducer, there is aspecified temporal frequency (f) such that f is equal to υ*v. Therefore,the spatial frequency relationship between the data marks and the clockreference structure is preserved as a temporal frequency relationshipbetween the data signal and the clock reference signal.

In prior art clock generation schemes that use synchronization fields insector headers, the separation of the data signal and the clock signalis realized by spatially alternating the data and clock signals. Theseparation is accomplished by only re-synchronizing the write clockduring the sector headers and running the write clock open loop whilethe optical transducer is coupled to data fields of the optical disk.The spatial multiplexing becomes time domain multiplexing as sectorheaders and data fields alternately pass the optical transducer of anoptical disk reader or optical disk recorder.

Spatial multiplexing as previously described, can not be used to obtaina clock reference structure which is coincident with the data structure.Rather, it is necessary that the clock reference signal be separablefrom the data signal while data is being read or written. Generally,there are three optical storage configurations available foraccomplishing the required separation of the clock reference signal andthe data signal.

The first configuration includes the clock reference structure having aspatial frequency at which the spatial frequencies of the data have beenspecifically encoded to be nulled. FIG. 13 shows the frequency spectrumof the first configuration. That is, the data stored on the optical diskis encoded so that the data spatial frequency spectrum 1110 does notinclude appreciable signal power at the clock reference structurespatial frequency 1112. This is similar to the configurations describedby U.S. Pat. Nos. 4,238,843, 4,363,116, 4,366,564, 4,375,088, 4,972,401.In this configuration, the data decoding is designed to ignore datafrequency components which are at the clock reference frequency. Theprimary disadvantage of this configuration is that special coding of thedata is required. The data coding required for this configuration isincompatible with several existing coding standards including theDVD-ROM standard.

A second configuration includes the clock reference structure having aspatial frequency which is greater than the spatial frequency of thedata. FIG. 14 shows the frequency spectrum of the second configuration.The data is stored on the optical disk so that the data spatialfrequency spectrum 1210 is lower than the clock reference structurespatial frequency 1212. For this configuration, an optical reader like aDVD-ROM optical reader, can not detect the spatial frequency of theclock reference structure. That is, the optical resolution of theoptical disk writer is greater than the optical resolution of theoptical disk reader. The optical disk writer can acquire and isolate theclock reference signal while excluding the data signal due topre-existing data using well known signal processing techniques.

A third configuration includes the clock reference structure having aspatial frequency which overlaps the spatial frequency spectrum of thedata. FIG. 13 shows the frequency spectrum of the third configuration.The data is stored on the optical disk so that the data spatialfrequency spectrum 1310 overlaps the clock reference structure spatialfrequency 1312.

The first configuration constrains the coding of data stored on theoptical disk to an extent that this configuration can not be used forwriting data to optical disks which are to be read by a DVD-ROM reader.

The second and third configurations are the subject of the invention.The second configurations offers the advantage that the clock referencefrequency is greater than the clock reference frequency of the thirdconfiguration. As previously described, the greater the frequency of theclock reference signal, the lower the amount of jitter that will beadded to the write clock.

The optical disk of the present invention includes construction forproducing a data signal with a high signal-to-noise ratio (SNR) whenread by an optical disk reader. A clock reference signal is an unwantedsource of noise if it appears within the data signal of the reader. Theoptical and electronic specifications of the optical transducer of astandard DVD optical disk reader are defined by a DVD formatspecification and are well known. Further, DVD readers are publiclysold. It is possible to determine the extent a clock reference structureformed on a re-writable DVD optical disk produces noise in the datasignal of a standard DVD optical disk reader. The optical disk of thepresent invention includes construction for minimizing such noise.

The optical disk of the invention further includes construction toproduce a high SNR clock reference signal while being recorded by anoptical disk recorder of the invention. A data signal produced bypre-existing data marks on the disk is an unwanted source of noise ifthe data signal appears within the clock reference signal of therecorder. The optical disk recorder of the invention includesconstruction to maximize the clock reference signal while minimizingnoise due to pre-existing data marks.

The clock reference structure of the re-writable optical disk of theinvention is described by way of three embodiments. Each embodimentsubstantially eliminates a clock reference signal as a potential noisesource in the data signal produced by a standard optical disk reader,and produces a high SNR clock reference signal in an optical diskrecorder of the invention. Each of the embodiments enables the opticaldisk to be recorded in DVD format and subsequently re-written, in wholeor in part, such that the disk is readable by a standard DVD reader.

FIG. 16 depicts a first embodiment clock reference structure comprisinga series of clock pits 33 arranged to form a servo track having a centerline 37 on the recording layer of the optical disk. The clock referencestructure has a fundamental spatial frequency defined as the reciprocalof the spatial period 35 measured between the centers of adjacent clockpits. The spatial frequency of the clock reference structure is higherthan the cutoff frequency of the optical transducer of a standardoptical disk reader. Therefore, the clock reference signal will notappear in the data signal produced by the optical disk reader. In thisembodiment, the clock pits comprising the clock reference structure alsoperform the functions of a servo track. Data marks 19 are recorded alongthe servo track.

In an optical disk recorder of the invention, the undesired data signalcan be separated from the clock reference signal electronically. Thefrequency of the clock reference signal exceeds the highest fundamentalfrequency of the data signal, permitting the data signal to besubstantially removed by high-pass electronic filtering. Electronicsignal separation is enhanced due to the high spectral power and narrowspectral bandwidth of the clock reference signal.

In an optical disk recorder, significant additional rejection of thedata signal is obtained by detecting the clock reference signal splitdetection (sometimes called tangential push-pull detection), an opticaldetection method well known in the art. In FIG. 10, an optical detector114 is located substantially at a pupil of the optical transducer (anoptical location sometimes referred to as “in the far field of thedisk”). The lens 194 forms a pupil at detector 114 by forming an imageof aperture stop 99 on detector 114. FIG. 17 shows an enlarged plan viewof optical detector 114. The circular perimeter 31 shows the outerboundary of the area of detector 114 that is illuminated by beam 106 ofFIG. 10. Line 29 indicates the tangential direction relative to the disk(servo tracks are parallel to the tangential direction). The detector114 is symmetrically divided into four detection areas called quadrants.Each quadrant produces an electrical output signal which issubstantially proportional to the optical power incident on thatquadrant, as is well known in the art. As shown in FIG. 17, detectorquadrants 21, 23, 25 and 27 produce electrical output signals A, B, C,and D, respectively. A split detection signal is produced by combiningthe signals from the detector quadrants according to the formula((A=D)−(B=C)), which is sometimes normalized by dividing by (A=B=C=D).The theory of optical disk readout has been extensively studied, and thecharacteristics of signals produced using split detection are well knownfor a variety of structures on the recording layer.

Split detection produces substantially no signal from the data marksproduced by phase change recording wherein data marks primarily affectthe amplitude of the reflected light but not its phase. (Note that thename “phase change” applies to the crystalline or amorphous phase of therecording layer, not whether the recorded marks affect the amplitude orphase of the incident light.) The clock pits which constitute the clockreference structure produce a well-modulated clock reference signal whendetected using split detection in the transducer of the optical diskrecorder. For best SNR of the clock reference signal, the preferredround trip optical depth for the pits is λ/4, where λ is the wavelengthof the light used in the optical transducer of the optical diskrecorder. The optical depth of structures on the recording layer of anoptical disk is defined as physical depth multiplied by the refractiveindex of the disk substrate material in contact with the recordinglayer.

Referring to FIG. 18, curve 123 shows the MTF for an optical transducerwhich produces a clock reference signal using split detection. Splitdetection causes a reduction in MTF at low spatial frequencies, but doesnot reduce the cutoff frequency or the MTF of the optical transducer ofthe recorder at the clock reference structure spatial frequency 121.Split detection therefore provides a means for producing awell-modulated high-resolution clock reference signal in the recorderwhile writing or re-writing data on an optical disk of the invention.

In a second embodiment clock reference structure, as depicted in FIG.19, servo tracks comprise grooves 3 in the recording layer and the clockreference structure comprises edges of grooves 5, 7 which oscillatesubstantially 180 degrees out of phase. For best clock reference signalSNR, the preferred round-trip optical depth of the grooves is λ/4 whereλ is the wavelength of the light used in the optical transducer of therecorder. Data marks 19 are recorded along servo tracks.

A prior art standard optical disk reader generates a data signal usingcentral aperture (CAP) detection. CAP detection is a method well knownin the art which forms a signal by summing the four quadrant signalsproduced by a quadrant detector similar to the optical detector 114 ofFIG. 17. The CAP detection signal is the (A=B=C=D) where A, B, C, Drepresent the signals from the detector quadrants. Alternatively, adetector with a single detection area large enough to capture the entirebeam diameter is equivalent and may be used. CAP detection is well knownin the art to have low sensitivity to structures on the recording layerhaving a round trip optical depth of λ/4 where λ is the wavelength ofthe light used in the optical transducer of the reader. This signalrejection characteristic of CAP detection permits the use of clockreference structures such as those shown in FIG. 19 with spatialfrequencies below the cutoff frequency of the optical transducer of astandard optical disk reader.

In a preferred configuration, however, the spatial frequency of theclock reference structure exceeds the cutoff frequency of the opticaltransducer of a standard optical disk reader. In this case, the clockreference signal will be entirely eliminated from the data signalproduced by the standard optical disk reader.

In an optical disk recorder, constructed to record data on disks havinga reference structure comprising groove edges that oscillatesubstantially 180 degrees out of phase, the preferred method fordetecting the clock reference signal is split detection, as previouslydescribed herein. As previously described, split detection producessubstantially no signal from data marks, such as phase change marks,which primarily affect the amplitude of the reflected light. Aspreviously noted, and as shown by curve 123 in FIG. 18, split detectionmaintains the full MTF and cutoff frequency of the optical transducer ofthe recorder at the clock reference signal spatial frequency 121,enabling a well-modulated clock reference signal to be produced.

In a third embodiment clock reference structure, as depicted in FIG. 20,servo tracks comprise grooves 3 in the recording layer, the clockreference structure comprising edges 5, 7 of grooves which oscillate inphase. The preferred round-trip optical depth of the grooves is λ/4.Data marks 19 are recorded along servo tracks. As previously discussed,a standard optical disk reader uses central aperture (CAP) detection forgenerating a data signal. It is well known in the art of optical datastorage that CAP detection substantially does not detect a signalproduced by groove edges that oscillate in phase. It is also well knownthat CAP detection has very low sensitivity to structures having a roundtrip optical depth λ/4. These two modes of signal rejection worktogether to enable the spatial frequency of the clock referencestructure to be below the cutoff frequency of the optical transducer ofa standard DVD reader without producing unacceptable levels of noise ina data signal produced by the reader.

In an optical disk recorder, constructed to record data on disks havinga reference structure comprising groove edges that oscillate in phase,the preferred method for detecting the clock reference signal is radialpush-pull detection, an optical method well known in the are of opticaldata storage. Radial push-pull detection forms a signal according to theformula ((A=B)−(C=D)), which is sometimes normalized by dividing by(A=B=C=D). As previously discussed, A, B, C and D are electrical outputsfrom quadrants 21, 23, 25 and 27 of detector 114 in FIG. 17. Radialpush-pull detection produces substantially no signal from data marks.Data marks are not detected, first because they primarily affect theamplitude of the reflected light, and secondly because they arenominally symmetric about the center of the track. As is well known inthe art, radial push-pull detection is sensitive to structures whichaffect the phase of the reflected light and which are asymmetric abouttrack center. Radial push-pull detection produces a well-modulatedsignal from groove edges which oscillate in phase, especially when theround trip optical depth of the groove is λ/4. Radial push-pulldetection produces a well-modulated signal from groove edges whichoscillate in phase, especially when the round trip optical depth of thegroove is λ/4. Radial push-pull detection provides sufficient rejectionof the undesired data signal to permit recovery of a clock referencesignal having frequency within the frequency range of the data. It isdesirable to provide the ability to use a clock reference structure witha spatial frequency below the cutoff frequency of a standard opticaldisk reader because the radial push-pull signal detection method reducesthe cutoff frequency of the recorder's optical transducer whenrecovering a clock reference signal. Curve 125 of FIG. 18 illustratesthe MTF for an optical transducer in an optical disk recorder duringdetection of a clock reference structure using radial push pulldetention. The cutoff frequency (f_(c)) 127 of the optical transducer ofthe optical disk recorder for purposes of detecting a clock referencestructure is reduces from a value of 2NA/λ to a value of:f_(c)=√{square root over ((2NA/λ)²−(1/P²))}{square root over((2NA/λ)²−(1/P²))}

Where the track pitch P is the radial distance between track centers.The MTF curves of FIG. 18 have been derived for the same opticaltransducers that are represented by MTF curves 116 and 118 of FIG. 11.Curves 116 and 118 are shown again as dotted curves in FIG. 18. In FIG.18, the MTF of the optical transducer in an optical disk recorder isrepresented by curve 118. When a clock reference signal is detectedusing radial push pull detection, the MTF of the optical transducer isreduced. Curve 125 shows the reduced MTF. The cutoff frequency has alsobeen reduced, from 2.46 cycles/um for curve 118 to 2.06 cycles/um forcurve 125. This MTF decline associated with radial push pull detectionsignificantly reduces the modulation of a clock reference signal havinga spatial frequency above the cutoff frequency of a standard opticaldisk reader. For this reason, the preferred configuration of thisembodiment uses a clock reference structure having a spatial frequencybelow the cutoff frequency of a standard optical disk reader. Note thatthe MTF reduction illustrated with reference to FIG. 18 applies only tothe detection of the clock reference structure and does not affect theresolution of the optical transducer for the purpose of recording data.

Note that the radial push pull signal contains tracking errorinformation at frequencies substantially below the clock referencesignal frequency and may also be used generate a tracking error signalfor use by a tracking positioner.

The invention can include other clock reference structures such as aclock reference structure which consists of a groove having a singleedge which oscillates. The three clock reference structures describedhere are by way of example.

FIG. 21 shows another embodiment of the optical disk recorder of theinvention which includes a second optical transducer 182 for readingdata stored on an optical disk 80. The optical disk recorder 81 has afirst optical transducer 82 and a second optical transducer 182 whichare optically coupled to the recording layer of the optical disk 80. Thefirst optical transducer 82 is used for recording data and operates aspreviously described with reference to FIG. 10. The second opticaltransducer 182 follows a servo track as the optical disk 80 rotates. Thedata marks cause the second optical transducer 182 to produce a datasignal as the optical disk 80 rotates. The second optical transducer 182includes several optical devices and has many similarities with opticaltransducer 82. A laser 190 emits a linearly polarized beam of light 192which is collimated by a collimator lens 194. The light beam 192 passesthrough a polarization beam splitter 196. The light beam 192 isconverted from linear polarization to circular polarization by a quarterwave retardation plate 198. The light beam 192 is focused by anobjective lens 200 onto the recording layer of the optical disk 80containing recorded data marks. A portion of the light beam 192 isreflected by the optical disk 80 and returns through the objective lens200 and the quarter wave plate 198. Upon passing back through thequarter wave retardation plate 198, the light beam 192 is again linearlypolarized. However the polarization direction of the light beam 192 isrotated 90 degrees relative to its initial orientation. Therefore, thepolarization beam splitter 196 reflects substantially all of light beam192 towards beam splitter 202. The beam splitter 2020 splits the beam192 into a first light beam 204 and a second light beam 206. The firstlight beam 204 is collected by a first light beam 204 and a second lightbeam 206. The first light beam 204 is collected by a first lens 208 ontoa first detector 210 which is arranged to produce a focus-error signal.The second light beam 206 is collected by a second lens 212 onto asecond detector 214 which is arranged to produce a data signal. Thesecond detector 214 also produces a tracking-error signal used by atracking positioner. Detectors 210 and 214 generally include multipledetection areas and produce multiple detection signals as is well knownin the art. Many alternative arrangements of the optical components anddetectors are possible, including arrangements which combine oreliminate optical components shown in FIG. 21.

FIG. 22 illustrates another embodiment of the optical disk recorder ofthe invention. This embodiment includes another configuration of asecond optical transducer 282 for reading data stored on an optical disk80 and uses the same objective lens 100 as optical transducer 82 whichis used for recording data. The shared objective lens is referred to asa combination objective lens 100. FIG. 22 shows the optical diskrecorder 81 in which the second optical transducer 282 is opticallycoupled to data marks on the recording layer of optical disk 80. Thesecond optical transducer follows a servo track as the optical diskrotates. The data marks cause the second optical transducer to produce adata signal as the optical disk rotates. As illustrated in FIG. 22, thesecond optical transducer 282 includes several optical devices and hasmany similarities with optical transducer 82. A laser 290 emits alinearly polarized beam of light 292 which is collimated by a collimatorlens 294. The light beam 292 passes through a polarization beam splitter296. The light beam 292 is converted from linear polarization tocircular polarization by a quarter wave retardation plate 298. The lightbeam 292 then passes through an aperture stop 99 and is focused by anobjective lens 100 onto the recording layer of the optical disk 80containing recorded data marks. A portion of the light beam 292 isreflected by the optical disk 80 and returns through the objective lens100 and the quarter wave plate 298. Upon passing back through thequarter wave retardation plate 298, the light beam 292 is again linearlypolarized. However the polarization direction of the light beam 292 isrotated 90 degrees relative to its initial orientation. Therefore, thepolarization beam splitter 296 reflects substantially all of light beam292 towards beam splitter 302. The beam splitter 302 splits the beam 292into a first light beam 304 and a second light beam 306. The first lightbeam 304 is collected by a first lens 308 onto a first detector 310which is arranged to produce a focus-error signal. The second light beam306 is collected by a second lens 312 onto a second detector 314 whichis arranged to produce a tracking-error signal used by the trackingpositioner, and a data signal containing information encoded in datamarks on optical disk 80. Detectors 310 and 314 generally includemultiple detection areas and produce multiple detection signals as iswell known in the art. Many alternative arrangements of the opticalcomponents and detectors are possible, including arrangements whichcombine or eliminate optical components shown in FIG. 22. The laser 290emits light at a longer wavelength than the laser 90. The beam splitter296 is a wavelength sensitive beam splitter (sometimes called a dichroicbeam splitter) which transmits light of a first wavelength and reflectslight of a second wavelength. The shorter wavelength laser 90 of opticaltransducer 82 provides a smaller focused spot of light and acorrespondingly higher MTF and cutoff frequency for recording data marksand producing a clock reference signal. The longer wavelength laser 292of second optical transducer 282 provides a larger focused spot and acorrespondingly lower MTF and cutoff frequency for reading data marks.

Another embodiment of the invention uses a variation of the componentsshown in FIG. 10 and previously described. As shown in FIG. 10, anoptical transducer 82 performs the functions of both an optical diskrecorder and an optical disk reader. When used as an optical diskrecorder, the components of the optical transducer 82 perform aspreviously described with reference to FIG. 10. When used as an opticaldisk reader, the optical transducer 82 is optically coupled to datamarks on the recording layer of optical disk 80. The optical transducer82 follows a servo track as the optical disk 80 rotates. The data markscause the optical transducer 82 to produce a data signal as the opticaldisk 80 rotates. The laser 90 emits a linearly polarized beam of light92 which is collimated by a collimator lens 94. The light beam 92 passesthrough a polarization beam splitter 96. The light beam 92 is convertedfrom linear polarization to circular polarization by a quarter waveretardation plate 98. The light beam 92 then passes through an aperturestop 99. The aperture stop 99 is dynamically controlled to be smallerwhen the optical transducer 82 is used as an optical disk reader andlarger when the optical transducer 82 is used as an optical diskrecorder. When the diameter of the aperture stop 99 is reduced, theeffective numerical aperture of objective lens 100 is reduced. The lightbeam 92 passes through the objective lens 100 and onto the recordinglayer of the optical disk 80 containing recorded data marks. The MTF andthe cutoff frequency of optical transducer 82 are reduced when thediameter of aperture stop 99 is reduced and a data signal is producedthat does not contain unwanted noise produced by a clock referencestructure formed on the recording layer of the optical disk 80. Aportion of the light beam 92 is reflected by the optical disk 80 andreturns through the objective lens 100 and the quarter wave plate 98.Upon passing back through the quarter wave retardation plate 98, thelight beam 92 is again linearly polarized. However, the polarizationdirection of the light beam 92 is rotated 90 degrees relative to itsinitial orientation. Therefore, the polarization beam splitter 96reflects substantially all of light beam 92 towards beam splitter 102.The beam 102 splits the beam 92 into a first light beam 104 and a secondlight beam 106. The first light beam 104 is collected by a first lens108 onto a first detector 110 which is arranged to produce a focus-errorsignal. The second light beam 106 is collected by a second lens 112 ontoa second detector 114 which is arranged to produce a tracking-errorsignal used by the tracking positioner. During data detection, detector114 is also arranged to produce a data signal containing informationencoded in data marks on optical disk 80. Detectors 110 and 114generally include multiple detection ares and produce multiple detectionsignals as is well known in the art. Many alternative arrangements ofthe optical components and detectors are possible, includingarrangements which combine or eliminate optical components shown in FIG.10. When adjusted to a higher effective numerical aperture for recordingdata, the optical transducer 82 provides a smaller focused spot of lightand a correspondingly higher MTF and cutoff frequency necessary forrecording data marks and producing a clock reference signal. Whenadjusted to a lower effective numerical aperture, the optical transducer82 provides a larger focused spot and a correspondingly lower MTF andcutoff frequency necessary for reading data marks.

Although specific embodiments of the invention have been described andillustrated, the invention is not to be limited to the specific forms orarrangements of parts so described and illustrated. The invention islimited only by the claims.

What is claimed:
 1. An optical disk comprising; a recording layer havingservo tracks; and a clock reference structure formed along the servotracks, the clock reference structure permitting data marks to bewritten and re-written to the recording layer in data fields ofindeterminate length, the reference clock structure permitting thegeneration of a clock reference signal which controls where first andsecond transition edges of data marks are written to the recording layerwith sub-bit accuracy.
 2. The optical disk as recited in claim 1,wherein the clock reference structure comprises a reference spatialfrequency which is greater than a predetermined spatial frequency. 3.The optical disk as recited in claim 2, wherein the predeterminedspatial frequency is the maximum spatial frequency detectable by astandard DVD-ROM reader.
 4. The optical disk as recited in claim 2,wherein the clock reference structure comprises edges of grooves of theservo tracks which oscillate in-phase at an oscillation spatialfrequency, the oscillation spatial frequency corresponding to thereference spatial frequency.
 5. The optical disk as recited in claim 2,wherein the clock reference structure comprises edges of grooves of theservo tracks which oscillate substantially 180 degrees out-of-phase atan oscillation spatial frequency, the oscillation spatial frequencycorresponding to the reference spatial frequency.
 6. The optical disk asrecited in claim 2, wherein the clock reference structure comprises pitsformed along the servo tracks, the reciprocal of a distance betweencenters of adjacent pits corresponding to the reference spatialfrequency.
 7. The optical disk as recited in claim 1, wherein a firstoptical transducer coupled to the clock reference structure generates aclock reference signal comprising a clock reference signal frequency. 8.The optical disk as recited in claim 7, wherein the first opticaltransducer coupled to data marks on the recording layer generates a datasignal having a frequency spectrum in which all fundamental frequencycomponents of the frequency spectrum are less than the clock referencesignal frequency.
 9. The optical disk as recited in claim 8, wherein astandard DVD-ROM reader can read the data marks but cannot detect theclock reference structure.
 10. An optical disk recorder comprising: anoptical disk rotatably mounted on the recorder, the optical disk havinga recording layer containing servo tracks; a first optical transduceroptically coupled to the recording layer of the optical disk, the firstoptical transducer following a servo track as the optical disk rotates;a clock reference structure formed along the servo tracks providing datafields of indeterminate length, the clock reference structure causingthe first optical transducer to produce a clock reference signal as theoptical disk rotates; means for recording data marks on the recordinglayer of the optical disk, wherein the data marks are recorded toinclude fundamental spatial frequencies less than a predeterminedspatial frequency; and a write clock which determines the placement offirst and second transition edges of data marks on the recording layerof the optical disk with sub-bit accuracy, the write clock being phaselocked to the clock reference signal.
 11. The optical disk recorder asrecited in claim 10, wherein the predetermined spatial frequency is thegreatest spatial frequency detectable by a standard DVD-ROM reader. 12.The optical disk recorder as recited in claim 10, wherein the servotracks include grooves and the clock reference structure comprises edgesof the grooves which oscillate in-phase.
 13. The optical disk recorderas recited in claim 12, wherein data marks cause the first opticaltransducer to produce an unwanted data signal as the optical diskrotates, and the clock reference signal is separated from the unwanteddata signal by detecting the clock reference signal using radialpush-pull detection.
 14. The optical disk recorder recited in claim 10,wherein the servo tracks include grooves and the clock referencestructure comprises edges on the grooves which oscillate substantially180 degrees out-of-phase.
 15. The optical disk recorder recited in claim14, wherein data marks cause the first optical transducer to produce andunwanted data signal as the optical disk rotates, and the clockreference signal is separated from the unwanted data signal by detectingthe clock reference signal using split detection.
 16. The optical diskrecorder recited in claim 10, wherein the clock reference structurecomprises pits formed along the servo tracks.
 17. The optical diskrecorder as recited in claim 10, wherein the data marks are positionedalong the servo tracks according to a DVD-ROM standard.
 18. The opticaldisk recorder as recited in claim 10, wherein the data marks arearbitrarily coded.
 19. The optical disk recorder as recited in claim 10,further comprising a second optical transducer which is opticallycoupled to the data marks on the recording layer, the second opticaltransducer following a servo track as the optical disk rotates, the datamarks causing the second optical transducer to produce a data signal asthe optical disk rotates.
 20. The optical disk recorder as recited inclaim 19, wherein the first optical transducer comprises a first laserand a first objective lens and the second transducer comprises a secondlaser and a second objective lens.
 21. The optical disk recorder asrecited in claim 20, wherein a numerical aperture of the combinationobjective lens is adjustably controlled to be lower when reading datathan when recording data.
 22. The optical disk recorder as recited inclaim 20, wherein a numerical aperture of the combination objective lensis adjustably controlled to be lower when reading data than whenrecording data.
 23. The optical disk recorder as recited in claim 20,wherein a wavelength of the second laser is greater than a wavelength ofthe first laser.
 24. An optical disk recorder for receiving an opticaldisk which is rotatably mountable on the recorder, the optical diskcomprising a recording layer having servo tracks and a clock referencestructure having a spatial frequency which is greater than apredetermined spatial frequency, the clock reference structure beingformed along the servo tracks and providing data fields of indeterminatelength, the optical disk recorder comprising: a first optical transducerwhich can optically couple to a recording layer of the optical disk, thefirst optical transducer following the servo tracks as the optical diskrotates, the clock reference structure causing the first opticaltransducer to produce a clock reference signal as the optical diskrotates; means for writing data marks on the recording layer of theoptical disk; and a write clock which determines the physical placementof first and second transition edges of data marks written on therecording layer of the optical disk with sub-bit accuracy, the writeclock being phase locked to the clock reference signal.
 25. The opticaldisk recorder as recited in claim 24, wherein the predetermined spatialfrequency is the maximum spatial frequency detectable by a standardDVD-ROM reader.
 26. The optical disk recorder as recited in claim 24,wherein the first optical transducer can detect higher spatialfrequencies that an optical transducer of a standard DVD-ROM opticaldisk reader.
 27. The optical disk recorder as recited in claim 24,further comprising a second optical transducer which can opticallycouple to the data marks on the recording layer, the second opticaltransducer following a servo track as the optical disk rotates, the datamarks causing the second optical transducer to produce a data signal asthe optical disk rotates.
 28. The optical disk recorder as recited inclaim 24, wherein the first optical transducer comprises a first laserand a first objective lens and the second transducer comprises a secondlaser and a second objective lens.
 29. The optical disk recorder asrecited in claim 28, wherein a combination objective lens is both thefirst objective lens and the second objective lens and the objectivelens.
 30. The optical disk recorder as recited in claim 29, wherein anumerical aperture of the combination objective lens is adjustablycontrolled to be lower when reading data than when recording data. 31.The optical disk recorder as recited in claim 29, wherein a wavelengthof the second laser is greater than a wavelength of the first laser. 32.The optical disk as recited in claim 7, wherein the first opticaltransducer coupled to data marks on the recording layer generates a datasignal having a frequency spectrum in which the clock reference signalfrequency is within fundamental frequency components of the frequencyspectrum.
 33. The optical disk as recited in claim 32, further includingmeans for optically separating the data from the clock reference signal.34. The optical disk as recited in claim 32, further including means foroptically separating the clock reference signal the form the datasignal.
 35. An optical disk comprising; a recording layer having servotracks; a clock reference structure formed along the servo tracks, theclock reference structure permitting data marks to be written andre-written to the recording layer in data fields of indeterminatelength, the reference clock structure permitting the generation of aclock reference signal which controls where first and second transitionedges of data marks are written to the recording layer with sub-bitaccuracy; a first optical transducer coupled to the clock referencestructure generating a clock reference signal comprising a clockreference signal frequency; and wherein the first optical transducercoupled to data marks on the recording layer generates a data signalhaving a frequency spectrum in which the clock reference signalfrequency is within fundamental frequency components of the frequencyspectrum.
 36. An optical disk recorder comprising: an optical diskrotatably mounted on the recorder, the optical disk having a recordinglayer containing servo tracks, the servo tracks comprising grooves; afirst optical transducer optically coupled to the recording layer of theoptical disk, the first optical transducer following a servo as theoptical disk rotates; a clock reference structure comprising edges ofthe grooves which oscillate in-phase formed along the servo tracks, theclock reference structure providing data fields of indeterminate length,the clock reference structure causing the first optical transducer toproduce a clock reference signal as the optical disk rotates; means forrecording data marks on the recording layer of the optical disk, whereinthe data marks are recorded to include fundamental spatial frequenciesless than a predetermined spatial frequency; a write clock whichdetermines the placement of data marks on the recording layer of theoptical disk, the write clock being phase locked to the clock referencesignal; and wherein data marks cause the first optical transducer toproduce an unwanted data signal as the optical disk rotates, and theclock reference signal is separated from the unwanted data signal bydetecting the clock reference signal using radial push-pull detection.37. An optical disk recorder comprising: an optical disk rotatablymounted on the recorder, the optical disk having a recording layercontaining servo tracks, the servo tracks comprising grooves; a firstoptical transducer optically coupled to the recording layer of theoptical disk, the first optical transducer following a servo track asthe optical disk rotates; a clock reference structure comprising edgeson the grooves which oscillate substantially 180 degrees out-of-phaseformed along the servo tracks, the clock reference structure providingdata fields of indeterminate length, the clock reference structurecausing the first optical transducer to produce a clock reference signalas the optical disk rotates; means for recording data marks on therecording layer of the optical disk, wherein the data marks are recordedto include fundamental spatial frequencies less than a predeterminedspatial frequency; a write clock which determines the placement of datamarks on the recording layer of the optical disk, the write clock beingphase locked to the clock reference signal; and wherein data marks causethe first optical transducer to produce an unwanted data signal as theoptical disk rotates, and the clock reference signal is separated fromthe unwanted data signal by detecting the clock reference signal usingsplit detection.
 38. An optical disk, comprising: a recording layerhaving a servo track; a high spatial frequency clock reference structureformed along the servo track; and a data field on the recording layer,wherein newly written data marks to the data field overlap previouslywritten data marks in the data field when the data field isdiscontinuously written to the recording layer; wherein the spatialfrequency of the high spatial frequency clock reference structure isgreater than the spatial frequency spectrum of data in the data field.39. The optical disk of claim 38, wherein the optical disk does notcomprise synchronization fields.
 40. The optical disk as recited inclaim 38, wherein the data field comprises a plurality of data marks andeach data mark is positioned on the recording layer with sub-bitaccuracy.
 41. The optical disk as recited in claim 38, wherein thespatial period of the clock reference structure is a multiple of thechannel bit length.
 42. The optical disk as recited in claim 38, whereinthe servo track is shaped as a groove with first and second oppositelydisposed edges and further comprising track address information includedin the high spatial frequency clock reference structure as a low spatialfrequency modulation of the two oppositely disposed edges of the groove.43. An optical disk, comprising: a recording layer having a servo trackfor recording data fields of arbitrary length, wherein newly writtendata marks to the data field overlap previously written data marks inthe data field when the data field is discontinuously written to therecording layer; and a clock reference structure formed along the servotrack, the clock reference structure enabling writing of data on therecording layer, and enabling generation of a clock reference signalused for writing of the data; wherein the clock reference structureformed along the servo track comprises a first edge and a second edge ofa groove of the servo track, and track address information is includedin the clock reference structure as a low spatial frequency modulationof the edges of the groove.
 44. The optical disk of claim 43, furthercomprising a plurality of data marks written to the recording layer,wherein each data mark is positioned on the recording layer with sub-bitaccuracy.